This invention comprises a method and implementing apparatus to permit joining of metallic tubes, such as oil or gas pipelines, and oil well casings, without flame or arc welding. The invention is based upon use of the magnetic "pinch effect", sometimes called "magnetic pulse forming", to squeeze a joining tube of conductive metal around the ends of similarly electrically conductive or less electrically conductive pipes to be joined. With care in use, the method also can be used to join pipes of the same conductive material as the joining tube, or even tubes of non-conductive materials, such as ceramic ("tile") utilizing a conductive connection. The method also can be applied to produce neck-down or flare-up joints between tubes of different sizes.
In addition to being a flame hazard, the use of arc welding on steel pipes can produce serious stress micro-cracks that can form the basis for splits and ruptures when the metal is vibrated or stressed in the future. Some examples of disasters from this cause were experienced during the 1940's when arc welders used to "strike an arc" against the deck plates of a "Victory Ship" being constructed. The micro-cracks being later worked by the motion of the ship at sea, apparently caused several instances of tearing that nearly or completely destroyed the ships. This phenomenon apparently is worse when the arc welding is done in very cold weather, when the arc appears to instantly, in effect, anneal smaller width lines along the steel (versus the same situation in warm weather). In addition, arc welding tends to be non-uniform, and requires greater-operator skill and complicated post welding inspections.
In the oil drilling environment, it would be particularly advantageous to have a means for cold joining the sections of well casing pipes as they were successively sunk into the hole, without heat and without the need for accurately threaded fittings. Threaded fittings require some turning of the pipe sections in advance, or even on site. Although flame brazing and arc welding have been used to join pipes in the drilling field environment, there is often the risk of igniting a flame which is difficult to extinguish, because hydrocarbon gases tend to escape from the hole before any oil is found.
Similarly, large pipeline segments can be joined in the field without requiring the tedious and time-consuming efforts required by any class of welding, and this can be done in any kind of weather.
Long before the so called "heavy press" program or the explosive forming program for forming large objects, a still older method involved beating the soft metal against a sand bag, and repetitively trying its fit over the pattern. Although the results were acceptable, there also was a non-uniform distribution of work hardening stresses in the metal as the result of many, versus few, hammer blows on the various areas.
The magnetic pinch effect or "magnetic pulse forming" has for several decades been used to cause conductive metals, such as duraluminum to very rapidly (almost instantly) conform to a male pattern in order to produce deep draw forms, such as aircraft engine nacelles. This reduced the labor that formerly was involved in the repetitive hand forming and fitting of sections of sheet aluminum to a wooden, ceramic, plastic or plaster pattern.
Magnetic pulse forming, while requiring rather heavy and expensive equipment, is able to rapidly produce nearly identical draws to the same pattern with very little hand labor involved, and with the elimination of much of the human error that beset the earlier methods when used for certain classes of materials, and certainly with less expense and with greater convenience than the heavy presses or explosions which were previously involved.
Whenever a rapidly changing magnetic flux cuts across a conductive material, a current is induced within the material. This current is proportional to the initial intensity and time rate of change of the magnetic flux. The higher the rate of change, the greater the induced current.
Then, whenever there is an induced current, there is an associated magnetic field of such polarity as to oppose the magnetic field producing the current. This reactive force between the rapidly changing magnet and a metallic material within its field can produce very significant forces of repulsion. The effect sometimes is called "Lenz's Law Repulsion". One rather large-scale illustrative use of the effect is in the levitation of magnetically suspended railroad trains above the conductive tracks. In that case large alternating currents can be used in the coils to produce larger, repetitive changes in magnetic flux.
In magnetic pulse forming, a rapidly changing, unidirectional current is applied to the work coil from an energy storage capacitor. The required high rate of change of flux is produced by rapidly discharging a large electric charge from an energy storage capacitor through a very low resistance coil of a few turns. The initial discharge current can be extremely heavy and will rapidly decrease in the early part of its exponential decay curve. This heavy current, rapidly decaying, causes a rapidly decaying flux that induces a heavy, similarly decaying current within the metal of the work piece.
Because the flux is concentrated within the core of the work coil, the reaction flux from the induced current is directed radially outward against the flux of the work coil. This causes extreme forces of repulsion to "pinch" the work piece radially inward. (Of course, the work coil also, alternatively, can be placed within the work piece to produce a force that tends to swell the work piece radially outward toward an external form).
The basic principle of magnetic pulse forming is well known to the industry, but with the advent of heavy deep-draw presses, magnetic pulse forming tended to be less used in practice. However, as with any natural phenomenon, there are niches where it can become the method of choice in trade-offs versus the advantages and disadvantages of other methods of forming. The method herein described is such an application niche.